In parent applications Ser. Nos. 066,534 and 071,294 the quantity of mercury employed, while insufficient to form a pool regardless of the attitude of the relay, may be in excess of that found most desirable in order to assure that the contacts will never be dry. In the present application, devices are employed for minimizing the possibility of dry contacts forming, due to maldistribution of mercury when smaller amounts are employed. These devices include placing the armature so close to the enclosure walls that the walls are touched by the armature at each operation, thereby assisting mercury redistribution. This is particularly valuable if a non-mercury wettable enclosure is employed. Further, the armature may be secured to the enclosure at two diametrically opposed points only, or preferably, left floating in mercury, permitting it to translate nearly planarly, and thereby squeezing mercury fillets which always exist during operation. A helical capillary groove, to act as a wick for mercury, may be formed in the armature, and/or in the casing, to assist in distributing mercury. Alternatively, mercury wettable surfaces may be roughened, for the same purpose. In addition, the armature is elongated, and mercury possesses high electrical resistivity. To reduce internal resistance of the relay, the armature may be laminated to include material of lower resistivity than is possessed by mercury.
The initial impetus toward the relay of the present invention and many of its underlying concepts was provided in Donath, U.S. Pat. No. 3,144,533. That patent provides the concept of utilizing a thin layer of mercury in a mercury switch, which covers mercury wettable surfaces in the enclosure for the switch, but is inadequate to form a free flowing pool of mercury regardless of the attitude of the switch. That patent discloses three distinct species of its basic concept, two of which employ a slug, freely riding on a layer of mercury because it is unwettable by the mercury, and the other being a reed switch. It has been believed crucial to the Donath device to utilize a very small physical structure and large wettable areas relative to the size of the device, in order to solve a crucial problem, i.e., the maintenance of a liquid layer over long periods of time as the relay operates. Liquid on the switch contacts is continuously being lost by impact and for other reasons, and must be able to replenish itself automatically as a layer, if the switch is to remain operative and this must occur regardless of the attitude of the switch.
The present invention employs a metallic envelope, a stationary contact or contacts, anchored via a non-wettable insulator or insulators extending through a wall or walls of the envelope, and an armature in the form of a single spiral, such as is used as a mainspring in a watch. The turns of the spiral have spacings adapted to hold mercury, and the entire spiral is mercury wettable, and all the turns of the spiral interact with each other and with the mercury, and the surface may be grooved or roughened to facilitate flow of mercury. Both header and cap may be non-magnetic, or the header can be magnetic and the cap non-magnetic, or vice versa, or both header and cap can be magnetic if non-magnetic windows are provided in the magnetic element, e.g., at the insulators, to permit entrance of magnetic flux. It follows that the spiral provides a very considerable high surface tension reservoir of mercury as a continuous thin layer on its surface, and the layer tends to reintegrate itself when broken by operation of the relay. The spring can be small or large, and there is no longer a requirement that the switch be small, as in the Donath device. It can be made to handle 50.A of current if desired. The interior walls of the enclosure, which is in the form of a header and cap welded together, may be mercury wettable or not, except that the insulators which enable stationary contacts to feed through must be non-wettable. The spiral is physically secured at its perimeter to the envelope, either by welding or clamping or it may float in a circumferential securing slot, and the envelope can then constitute a switch output electrode, the spiral operating as a damped reed. Damping factors can be inserted for avoiding reed self-vibration. The very long reed spring can be made extremely flexible, so that in operation parts of it can impact against the envelope. This impact against the envelope has no effect on switching but does aid in redistributing mercury and is particularly valuable if the enclosure is non-wettable. In the present device the spring constitutes a major reservoir of mercury, and the structure lends itself to fabrication in relatively large diameter, thin envelopes, which are inexpensive to make because they can be welded in a gaseous atmosphere under pressure, and thus readily filled with gas, as hydrogen, at high pressure, say 200 - 250 pounds/square inch.
Diaphragm type switches which do not employ mercury are commercially available. These employ diaphragms which have circular slots or plural spiral slots. There are, however, no spiral slotted discs, having many turns, say three, presently employed in relays, so that the latter represent highly elongated, but compact reeds. The spring device or elongated reed of this invention can be advantageously utilized in a relay which duplicates the present relay except in that mercury is omitted, because the spring is remarkably flexible, and therefore responds remarkably rapidly, yet without oscillation, or with highly damped vibration, to small magnetic forces. Damping can be controlled in terms of spring design, mercury layer distribution, and the effect of the gas under pressure, or any of these.
The present invention, as applied in a mercury switch, requires that there be a layer of mercury on mercury wettable surfaces. The term "layer" may be distinguished from film and from pool.
A mercury film is one in which the position and shape of the liquid mercury do not change with respect to the solid. Films between relatively movable metal surfaces must be avoided since sticking results. A mercury layer is one in which the shape of the mercury changes but the mercury remains on average in contact with the solid, despite changes of attitude of the surface, or subjection thereof to shock or vibration. A mercury pool is one in which both the shape and location of the mercury change on a statistical and transient basis, the pool, at least as a whole, not being relatively permanently attached to a wettable surface.
When a mercury relay operates, mercury flies in all directions for each impact between the contacts. In addition, mercury is displaced due to forces, i.e., those of gravity, vibration and shock, temperature gradients and forces of surface tension. It is essential that a mechanism be present in a mercury relay for relocating mercury which has been displaced, to locations such that contacts will not be short circuited, or become dry, and in the present system this must occur regardless of switch attitude.
It can be shown by mathematical analysis that the available surface tension force to restrain a given volume of mercury on a mercury wetted surface is proportional to the length of the edge of the surface. It follows that breaking up a surface into small discrete areas increases the net surface tension forces over the total area, because it increases net length of edge; but perhaps more important, this facilitates flow of mercury so that it may very rapidly redistribute itself as a continuous layer when operation of the contacts ruptures the layer. A spiral spring has total edge length proportional to its length, but the effective edge length can be extended by slotting the turns and the cap and header inner surfaces can be improved in respect to surface tension if lined with spiral elements, either grooves or protuberances, or with fine mesh screening, or by merely roughening the surface which also assists in redistribution.
The advantages which the present system present over the structure of Donath is that a free slug is avoided, movement of the latter requiring considerable energy expenditure, and the only frictional forces which need be avoided are those internally of the spring and those due to viscosity of mercury and gas, leading to high sensitivity. Second, the enclosure of the present invention can be resistance welded closed, and this can be done in a 200 - 250 psi Hydrogen atmosphere, at low cost. The Donath device as presently designed cannot be welded closed, and hence cannot practically enclose high pressure gas. Third, the present device can be made large, and therefore capable of carrying high current, which is not true of the Donath unit. The Donath unit is a high precision unit, in terms of fabrication techniques, and therefore tends to be expensive. The present unit is extremely inexpensive because it lends itself to mass production by welding of the header to the cap in one operation.
It is found that the switch of the invention can have high resistance when conductive, because mercury has high resistivity and the reed involved is about 11/2 inches long. The mercury path along the reed, according to the present invention, is shunted by the reed itself, which is laminated to include a layer or layers of higher conductivity material than the conductivity of mercury.
Problems exist in respect to sticking of the armature to the stationary contact. This is avoided according to the teaching of the Bitko applications for United States patent, supra. The device of the present application also meets the problem of securing the armature within its enclosure, by permitting it to float on layers of mercury, which reduces welding problems. The present application also concerns itself with aiding redistribution of mercury by utilizing one or more of the following features: (1) adding a spiral capillary slot to the turns of the spiral; (2) adding spiral capillary slots to the inner surfaces of the enclosure; (3) adding projections to the inner surface of the enclosure against which the armature may impinge; (4) spacing the armature from the cap and/or header so closely that in the operated condition mercury continuously contacts the cap and/or header; (5) roughening mercury wettable surfaces, e.g., the armature.
In the prior art surfaces were rendered mercury wettable by coating those surfaces with platinum. Platinum is an expensive metal, and substitution is therefore desirable. Nickel is known to be mercury wettable, but it readily oxidizes, and when oxidized is no longer wettable. This renders nickel plating, to achieve wettable surfaces, undesirable. It has been found that if nickel is combined with phosphorous it provides a hard stable surface which is more wettable than platinum, but which is also very hard and smooth. The addition of phosphorous, perhaps of the order of 5-10%, to nickel, changes the surface properties of the nickel so that even in the presence of oxygen the nickel remains highly mercury wettable. The nickel-phosphorous combination may be achieved either by electroless plating, which in the usual commercial process results in the incorporation of phosphorous into the nickel, or by electrolytic plating, in which case suitable phosphorous compounds must be incorporated in the usual nickel chloride plating bath.